1. Field of the Invention
The present invention relates to a sintered silicon nitride having high strength and toughness, which is applicable to parts required to possess heat resistance, corrosion resistance, wear resistance, etc. such as, for instance, engine or sliding parts.
2. Prior Art
Silicon nitride (Si.sub.3 N.sub.4) broken down into one of high-temperature structural ceramic materials is one of attractive ceramic materials that have sufficient strength, are chemically stable and resistant to thermal impact at high temperatures.
Japanese Patent Kokai Publication No. 21254/1988 discloses one typical method for making such silicon nitride ceramics with the use of a mixture of silicon nitride powders having a relatively large particle size with those having a relatively small particle size as a raw powdery material, wherein a sintering aid is previously added to the silicon nitride powders having a larger particle size, which are then mixed with the silicon nitride powders having a smaller particle size to obtain a mixture, and that mixture is compacted and sintered.
Such a conventional method for making sintered silicon nitride takes aim at obtaining uniform sintered pieces by improving the formability of a raw powdery material by the incorporation of silicon nitride powders differing in particle size, thereby increasing the compact's density.
However, although the sintered pieces obtained by this method has relatively high mechanical strength, their bending strength and toughness are still far from satisfactory.
Following the invention set forth in the above Kokai publication, we have made intensive and extensive studies of making sintered silicon nitride with starting materials having varied particle sizes, and have now found that the bending strength, toughness, etc. of sintered silicon nitride has a close relationship to the proportion, particle size and particle-size distribution of .beta.-type silicon nitride.
Such findings underlie the present invention.
A main object of the present invention is to provide a high-density sintered silicon nitride excelling in such mechanical properties such as bending strength and fracture toughness.